The present invention relates to platinum catalyzed SiH-olefin silicone compositions utilizing an addition cure reaction and more specifically it relates to an improved process for formulating such silicone rubber products. The present invention allows a silicone formulator or fabricator to selectively control the rheological properties of the silicone products and process. Additionally, the present invention provides a silicone rubber product which utilizes an effective level of inhibitor which is lower than that which was previously available in the art.
In U.S. Pat. No. 4,061,609, issued Dec. 6, 1977, Bobear demonstrated a silicone rubber composition which has been shown to be useful, commercially successful, and which has met with wide acceptance in the silicone industry. This patent is hereby incorporated by reference.
Bobear recognized that several major disadvantages of prior art silicone rubber compositions could be eliminated entirely with the use of a proper inhibitor for the platinum catalyzed cure reaction. SiH-olefin platinum catalyzed compositions had been well-known in the art. Such compositions generally comprise a vinyl-containing polysiloxane base material having a treated or untreated filler therein and a hydrogen-containing polysiloxane along with a platinum catalyst which could be solid platinum metal deposited on a solid carrier such as gamma alumina or it could be a solubilized platinum complex. Normal procedure was to package the vinyl polysiloxane, the filler and the platinum catalyst in one package and to provide a second package containing the hydrogen-containing polysiloxane. The fabricator or other user of the material produced a cured silicone elastomer by mixing the two packages according to specified proportions whereupon the composition could be fabricated to a desired shape and allowed to cure either at room temperature over a period of time or at elevated temperatures in relatively very short periods of time.
The above-described compositions which were sold in the two component or package format are usually referred to as room temperature vulcanizable silicone rubber compositions and more specifically, SiH-olefin platinum catalyzed room temperature vulcanizable silicone rubber compositions. It is to be understood that these types of compositions could be cured at varying rates depending upon the temperature. For example, at room temperature the composition might take 1 hour to 12 hours to cure but at elevated temperatures such as 100.degree. to 200.degree. C. the composition might cure in seconds or minutes.
Such compositions start curing as soon as the two components are mixed together and will usually cure or at least set in approximately 1 hour even at room temperature. Therefore, it was desirable to incorporate into the prior art compositions inhibitors which would retard the curing of the composition for at least 12 hours when the two components were mixed together in order to allow the composition to be fabricated to the desired shape before such composition sets. After the two components have been mixed together but prior to their having set such that they cannot be molded further, it is desirable to have as long a work-life as possible. The function of the inhibitor is to increase the work-life of the composition prior to curing at an elevated temperature. The inhibitor must provide suitable work-life yet not impede or any way detract from the final cure and properties of the composition of the silicone elastomer.
Among the prominent prior art inhibitors were acetylenic-functional organic polymers and monomers as shown by Kookootsedes in U.S. Pat. No. 3,445,420.
These inhibitors were ultimately undesirable insofar as the acetylenic radical-containing compounds had to be sealed in air tight containers because exposure or leaks to the atmosphere will cause the acetylenic compound to evaporate thereby decreasing its inhibiting properties. This was a further disadvantage insofar as ordinarily SiH-olefin platinum catalyzed compositions did not otherwise have to be packaged in air tight containers.
Accordingly, Bobear recognized a very effective class of inhibitors utilizing hydroperoxy radicals which were quite effective and overcame many of the prior art disadvantages. As mentioned above, Bobear's rubber composition met with success in the market place and served to provide useful silicone rubber fabricated silicone rubber products. Not only did Bobear avoid the use of explosive acetylenic compounds which require careful manufacturing procedures for their preparation and use but he was able to provide inhibitors having a higher effective rate of inhibition.
Prior SiH-olefin platinum catalyzed compositions usually consisted of polysiloxane polymers having a viscosity of approximately 1000 to 500,000 centipoise at 25.degree. C. so that such polymers could be manipulated or worked at a rate which is more efficient than possible with higher viscosity polymers. In other words, the lower viscosity polymers serve the additional purpose of assisting in providing additional work-life.
Since Bobear utilized extremely effective inhibitors he was able to provide high viscosity SiH-olefin platinum catalyzed compositions wherein viscosity could range anywhere from one million to 200 million centipoise at 25.degree. C. Not only did these compositions remain workable, the final products exhibited very satisfactory higher tensile strengths. These compositions opened up entirely new markets and uses for these SiH-olefin platinum catalyzed silicone rubber compositions.
The difficulty in developing such high viscosity SiH-olefin platinum catalyzed compositions arose because they normally had to be worked on a mill or other apparatus after the two ingredients were mixed together thereby requiring extended work-life of at least 12 hours. In the past when such high viscosity materials were used, portions of the material would cure right on the mill upon contact and mixing of the two components. This made it exceedingly difficult to fabricate products. Bobear provided compositions which avoided many of these problems.
Fabricators and formulators in the silicone industry immediately recognized the significance of Bobear's contribution. It has become an industry standard to utilize Bobear's rubber composition in a two-package format at a 1:1 or 50:50 mixture ratio. This system works quite well as evidenced by the success in the market but there are several disadvantages which have now been overcome by the present invention.
The present invention provides a system for packaging and formulating silicone rubber products which lowers the cost to the formulator, especially inventory costs since large quantities of two different packages are no longer required. The process of the present invention also allows more reproducible results and products since formulators have previously been well accustomed to utilizing an approximately 99:1 system.
Furthermore, the present invention offers flexibility to the fabricator who may now vary the cure rate of the system thereby facilitating various types of fabrication such as extrusion through a hot air tunnel or a steam autoclave as well as molding via compression, tranfer or injection devices.
The flexibility afforded by the opportunity of varying the cure rates, as provided by the present invention can be demonstrated by considering various silicone rubber fabricating techniques. When the catalyzed composition is manipulated through an extrusion device and directed to a hot air vulcanization tunnel or zone, it is desirable that there be a relatively quick cure of the material. That is to say, best results are obtained if the composition begins to cure immediately upon exposure to the HAV zone.
However, if a hot mold technique is utilized such as compression or injection molding, a slower cure is necessary in order that the rubber first obtain the configuration of the molding device before the cure reaction is initiated.
A two-package, 50:50 mixture effectively fixes the ratio of catalyst and inhibitor and thereby limits a fabricator flexibility. Not only does the three-package system of the present invention provide such flexibility through the selective use of cure agents and cure modifiers, it does so at effective inhibitor levels which are lower than those previously provided by the art.
It is therefore a primary object of the present invention to provide a system for formulating silicone rubber products.
It is another object to provide silicone rubber compositions utilizing lower effective levels of inhibitors.
It is another object to provide a system for modifying the cure rate and rheological properties of a silicone rubber composition during the fabrication of such composition.
These and other objects will become apparent to those skilled in the art upon careful consideration of the following specification, examples and claims.